Bill Irwin's patch to avoid having to walk pagetables while generating
/proc/stat output.

It can significantly overstate the size of various mappings because it
assumes that all VMAs are fully populated.

But spending 100% of one of my four CPUs running top(1) is a bug.

Bill says this fixes a bug, too.  The `SIZE' parameter is supposed to
display the amount of memory which the process would consume if it
faulted everything in.  But "before it only showed instantiated
3rd-level pagetables, so if something within a 4MB aligned range hadn't
been faulted in it would slip past the old one".